1. Field of the Invention
This invention pertains to indicating instruments, particularly of the aircraft director type.
2. Description of the Prior Art
Aircraft flight director instruments are known that provide visual commands to the pilot indicating the control required to bring the craft to a predetermined flight path or attitude. A flight director instrument commonly utilized is the single cue type shown and described in U.S. Pat. No. 3,691,987, entitled "Indicator Mechanism for Navigation Instruments", by R. Strock, and assigned to the applications' assignee.
The single cue type of flight director indicator permits the utilization of a single indicating element that displays pitch and roll command motions that are realistically suggestive of the aircraft control required of the pilot in response to the visual commands. The single cue flight director of said U.S. Pat. No. 3,691,987 includes a drive mechanism having a first and second gear members mounted for rotation with respect to each other and with respect to the instrument housing. Drive means or servomotors are included for rotating the first and second gear members in the same direction and at the same angular velocity with respect to each other relative to the housing in response to a roll command signal and for rotating the members in opposition to each other in response to a pitch command signal. An indicator element is coupled to the first and second gear members so that a first motion is imparted thereto about the roll axis of the instrument when the first and second gear members rotate in the same direction and at the same angular velocity and a second motion is imparted thereto about the pitch axis of the instrument when one of the members rotates in opposition to the other.
An alternative prior art single cue type of flight director, described in the aforementioned Strock patent, may be realized by incorporating a platform in the instrument, the platform being mounted for rotation about the roll axis thereof. A meter movement which may be of the type disclosed in U.S. Pat. No. 3,577,195, and assigned to the applicants' assignee, or a servomechanism mounted on the platform provides pitch motions to an indicator element that is connected thereto. It is thus appreciated that visual roll commands are provided by rotating the platform, hence imparting roll motions to the indicator element. Energization of the meter movement or servomechanism mounted on the platform imparts the required pitch motions to the indicator. This prior art arrangement requires a large diameter swing circle for the platform as well as a counterweight for the meter movement or servomechanism, to reduce the power requirements of the roll servomotor, thereby increasing the dimensions required to house the instrument.
Accordingly, the practice of the aforementioned single cue flight directors require either a considerable number of gears or relatively heavy counterweights that require a large swing circle. For example, the drive mechanism of the flight director indicator disclosed in U.S. Pat. No. 3,691,987 requires a large number of spur gears, two ring gears and one mechanical differential. Moreover, the flight director indicator of the type having a relatively heavy servomechanism or meter movement, which applies a relatively large torque about the roll axis, would have its response and accuracy adversely affected by the torque if it were not for the counterweights. The required counterweights, precision gearing and mechanical differential of the prior art, increase both the weight and the cost of the flight director instrument. Since a decrease in weight, mechanical complexity as well as costs is especially desirable in aircraft flight instruments, it is particularly undesirable in such an environment to utilize devices having a considerable number of gears or counterweights. Accordingly, there is a desire and a need for a single cue flight director instrument having a reliable, compact and relatively inexpensive cue drive mechanism.